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Chlainomonas (Chlamydomonadales, Chlorophyta) is one of the four genera of snow algae known to produce annual pink or red blooms in alpine snow. No Chlainomonas species have been successfully cultured in the laboratory, but diverse cell types have been observed from many field‐collected samples, from multiple species. The diversity of morphologies suggests these algae have complex life cycles with changes in ploidy. Over 7 years (2017–2023), we observed seasonal blooms dominated by a Chlainomonas species from late spring through the summer months on a snow‐on‐lake habitat in an alpine basin in the North Cascade Mountains of Washington, USA. The Bagley Lake Chlainomonas is distinct from previously reported species based on morphology and sequence data. We observed a similar collection of cell types observed in other Chlainomonas species, with the addition of swarming biflagellate cells that emerged from sporangia. We present a life cycle hypothesis for this species that links cell morphologies observed in the field to seasonally available habitat. The progression of cell types suggests cells are undergoing both meiosis and fertilization in the life cycle. Since the life cycle is the most fundamental biological feature of an organism, with direct consequences for evolutionary processes, it is critical to understand how snow algal life cycles will influence their responses to changes in their habitat driven by climate warming. For microbial taxa that live in extreme environments and are difficult to culture, temporal field studies, such as we report here, may be key to creating testable hypotheses for life cycles. [ABSTRACT FROM AUTHOR]

The relative frequency of sexual versus asexual reproduction governs the distribution of genetic diversity within and among populations. Most studies on the consequences of reproductive variation focus on the mating system (i.e., selfing vs. outcrossing) of diploid‐dominant taxa (e.g., angiosperms), often ignoring asexual reproduction. Although reproductive systems are hypothesized to be correlated with life‐cycle types, variation in the relative rates of sexual and asexual reproduction remains poorly characterized across eukaryotes. This is particularly true among the three major lineages of macroalgae (green, brown, and red). The Rhodophyta are particularly interesting, as many taxa have complex haploid–diploid life cycles that influence genetic structure. Though most marine reds have separate sexes, we show that freshwater red macroalgae exhibit patterns of switching between monoicy and dioicy in sister taxa that rival those recently shown in brown macroalgae and in angiosperms. We advocate for the investigation of reproductive system evolution using freshwater reds, as this will expand the life‐cycle types for which these data exist, enabling comparative analyses broadly across eukaryotes. Unlike their marine cousins, species in the Batrachospermales have macroscopic gametophytes attached to filamentous, often microscopic sporophytes. While asexual reproduction through monospores may occur in all freshwater reds, the Compsopogonales are thought to be exclusively asexual. Understanding the evolutionary consequences of selfing and asexual reproduction will aid in our understanding of the evolutionary ecology of all algae and of eukaryotic evolution generally. [ABSTRACT FROM AUTHOR]

Determining the relative importance of sexual and asexual reproduction is critical for determining how genetic diversity is partitioned within and among populations. Macroalgae exhibit reproductive system variability, with many species capable of both sexual and asexual reproduction. Thus, rates of fragmentation (asexual) versus sporic recruitment (sexual) must be quantified. This is critical during range expansions when shifts to uniparental reproduction are common. Chondria tumulosa is a cryptogenic red alga that was discovered forming thick mats at Manawai, in the Northwestern Hawaiian Islands, during opportunistic sampling in 2019. It reduced the biodiversity and abundance of associated coral reef organisms. To characterize the reproductive system of C. tumulosa in this atoll, we visually assessed 124 thallus clumps for reproductive structures and used five microsatellite loci to genotype 41 thalli. All thalli bearing reproductive structures (20%) and all thalli that were genotyped were tetrasporophytes. No gametophytic thalli were observed using microscopy or microsatellite genotyping, though 80% of thalli observed under the microscope were vegetative. We observed in situ thallus fragmentation supported by the presence of 17 repeated genotypes and other genetic signatures, suggesting high rates of asexual reproduction. These results suggest that C. tumulosa is partially clonal in this atoll and has the potential for spreading to other nearby atolls. Rapid establishment, thallus fragmentation and tetrasporophytic dominance are consistent with other red macroalgal invasions studied to date. Understanding the reproductive system is an important component of developing management strategies for this alga and expanding our understanding of the population genetics of partially clonal taxa. [ABSTRACT FROM AUTHOR]

Haploid-diploid life cycles impose unique eco-evolutionary consequences, rendering commonly used proxies difficult to use (e.g. separate sexes prevent selfing). Population genetic analyses are therefore required to explore patterns of reproductive system variation. However, there are still few haploiddiploid species for which polymorphic, nuclear loci exist. This problem is particularly acute for algae. Here, we describe the development of the first microsatellite loci in a freshwater red alga. We tested 73 candidate loci against a panel of Batrachospermum gelatinosum (L.) De Candolle gametophytes that encompass much of its North American range. Ten loci consistently amplified and were characterized by clean peak architectures on a capillary sequencer with one allele per locus, as expected in a haploid gametophyte. We then explored some basic population genetic indices in gametophytes collected from one site and obtained good resolution based on the probability of identity (pid). Yet, we observed a pattern of clumped repeated genotypes throughout the stream reach sampled. The pattern of moderate genotypic richness could be due to intragametophytic selfing resulting in the complete loss of genetic diversity from a single gamete union. Future studies will need to sample more populations to determine if intragametophytic selfing is the dominant reproductive mode in this monoicous taxon. The loci developed here represent an important tool for studying freshwater red algal populations in specific as well as enhancing our understanding of reproductive system variation and the haploid-diploid life cycle of algae in general. [ABSTRACT FROM AUTHOR]